You guys need to make use of focus groups.
How would one go about that? I'm familiar with the term but generally it's been in connection with enormous corporations rounding up groups of people to take polls and various forms of A/B testing to figure out what color to make their new snack food logo, etc. I'm not saying it doesn't work for them (small attempt at humor aside), but I'm not sure what you mean by the term in our case.
FWIW over the years we have been putting a lot more effort (and money, and time) towards PR, marketing, and otherwise increasing the chances that our hard work won't miss 95% of its potential sales because everyone who looks at the store page bounces off the trailer or whatever. That includes making sure the idea is saleable before we really get into it. This is not our area of expertise, and frankly it's a problem I don't enjoy trying to solve (and have personally brushed off in the past as not-my-problem), but as you and others have pointed out it's a problem we have to solve if we want good financial results.
So perhaps focus groups are a next step on that path (hopefully that "the Dark Side" sign a few miles back was just joking), but some reference material would be helpful.
but he always seems to make the game that he wants to make instead
I can understand if it looks that way, but we are actually acutely aware of the tension between "what we'd like to do if it were 100% up to us" and "what we need to do to for financial reasons".
Shattered Haven was actually very cheap to make, compared to even Tidalis, because so much of the work had already been done back before AIW. So even though it really underperformed financially it wasn't actually a bad call (even in retrospect) to make that game. Whether it's a good call to make an expansion, that's a different question.
This is what happened with Tidalis, also. He was told multiple times that it was a match three in a sea of match three, and in spite of great reviews (it is a good game), it's not going to sell.
From what we've heard from other developers Tidalis actually did pretty well for a game of its genre. We just spent too much making it to reasonably expect a good return.
You have to decide between making the games that you want to make regardless of whether or not they sell, or working on focus groups and trying to find a happy medium between what gamers want to buy and what you want to make.
Believe it or not: we know